ABSTRACT Allogeneic hematopoietic stem cell transplantation (HSCT) is an established curative therapy for leukemia through the potent immune-therapeutic effect called the graft vs leukemia effect (GVL). However, utilization of allogeneic HSCT has been severely limited by its most significant complication, namely graft vs host disease (GVHD). Efforts to meaningfully separate GVHD from GVL in humans have been largely unsuccessful. An important concept in immunology is that cellular injuries cause releases of danger-associated molecular patterns (DAMPs), which are major causes of inflammation after HSCT. While several molecules and pathways that stimulate inflammation are increasingly appreciated, the role of pathways that are inhibitory or regulate responses to DAMPs have largely been underappreciated and not been targeted in clinical inflammatory or immune-pathologies. We have recently demonstrated promoting negative responses to DAMPs after experimental allo-HSCT by enhancing CD24-SiglecG interaction (Toubai et al, Blood, 2015) reduced experimental GVHD. But the key cellular and molecular pathways remain to be deciphered. Preliminary preclinical data experimental also demonstrate that enhancing this pathway with a novel CD24Fc fusion protein reduced experimental GVHD. Importantly, since then we have developed clinical grade CD24Fc fusion protein that shows safety in healthy human volunteers. These key preliminary data form the foundation for our central hypothesis that CD24Fc will reduce the severity of GVHD without affecting GVL. Thus, in this proposal we will build on these exciting preliminary data to bring together basic and clincial studies. We aim to translate basic observations into a hypothesis driven, proof of concept, first in human trial with CD24Fc protein and meld it with further dissection of the immune-biological mechanisms of the CD24-Siglec axis in negative regulation of allo-immunity. If successful, our proposal will lead to the development of a novel therapeutic strategy while simultaneously providing biological insights into GVHD. The specific aims (SA) of the proposal are: SA 1: Elucidate the cellular and molecular mechanisms of CD24-Siglec mediated regulation of GVHD In this SA we will test the hypothesis that binding of CD24Fc to the inhibitory Siglec-G receptors on both donor T cells and host APCs is critical for regulating DAMP mediated inflammation and aggravation of GVHD. SA 2: To perform a large, first in human clinical trial to determine whether administration of CD24Fc fusion protein to recipients of unrelated donor allogeneic HSCT following myeloablative condition will mitigate GVHD. We will explore the hypothesis that administration of CD24Fc fusion protein will be safe and reduce severe GVHD in a multicenter Phase IIa/b clinical trial.